Proper Estimate

Deck board calculator

Enter the deck size and the boards you plan to use. You get the rows, linear feet and boards to buy for each standard length, plus screws or hidden fasteners.

Deck

The way the boards run

Across the boards

Boards

Board width

Actual width. 5/4x6 and 2x6 boards are 5 1/2 in.

Gap between boards
Board length

10% for straight boards, 15% for diagonal

Joist spacing

For counting fasteners

Boards to buy

29

16 ft boards

  • Rows of boards26
  • Linear feet416 ft
  • With waste457.6 ft
  • Deck screws656 (2 boxes of 350)
  • Or hidden fasteners328 (4 boxes of 90)

By board length

  • 8 ft boards58 boards
  • 10 ft boards46 boards
  • 12 ft boards39 boards
  • 16 ft boards29 boards
  • 20 ft boards23 boards

How to work out deck boards

  1. Add the board width and the gap. A 5 1/2 in board with a 3/16 in gap covers 5.69 in of deck.
  2. Divide the deck width by that, and round up to whole rows. A 12 ft (144 in) wide deck takes 144 ÷ 5.69 = 25.32, so 26 rows.
  3. Multiply the rows by the deck length: 26 × 16 ft = 416 linear feet.
  4. Add waste and divide by the board length: 416 × 1.10 = 457.6 ft, and 457.6 ÷ 16 = 28.6, so buy 29 boards.

Whole rows matter: the last row still needs full-length boards even if you rip them narrower. That's why this calculator counts rows instead of dividing the deck's area by the board's.

Boards for common deck sizes

Deck size Linear feet 12 ft boards 16 ft boards 20 ft boards
10 × 10 ft 220 21 16 13
12 × 12 ft 312 29 22 18
16 × 12 ft 416 39 29 23
16 × 16 ft 544 50 38 30
20 × 12 ft 520 48 36 29
20 × 16 ft 680 63 47 38
20 × 20 ft 860 79 60 48

Length (the way the boards run) × width. 5 1/2 in boards, 3/16 in gaps, 10% waste, before any picture-frame border.

Choosing the board length

Boards as long as the deck mean no joints. On a longer deck, every butt joint has to land on a joist, and staggering the joints row to row looks better and is stronger. Pick the length that divides the deck with the least offcut: on a 20 ft deck, 20 ft or 10 ft boards waste nothing, while 16 ft boards leave 12 ft pieces you need to use elsewhere.

Gaps between boards

Wood moves with moisture. Wet pressure-treated boards are often laid tight or with a 1/8 in gap and open up as they dry. Kiln-dried wood and most composites need about 3/16 in to 1/4 in. Composite makers set the gap in their install guides, and hidden fastener clips usually set it for you.

Fasteners

Face-screwed boards take two screws at every joist they cross. Hidden fasteners take one clip at each joist crossing. The calculator adds 5% to both for drops and starter clips.

Questions

How many deck boards do I need for a 12 × 12 deck?

About 29 boards 12 ft long. With 5 1/2 in boards and 3/16 in gaps, 12 ft of width takes 26 rows; at 12 ft each that's 312 linear feet, plus 10% for waste.

How many deck boards do I need for a 20 × 16 deck?

With boards running the 20 ft length: 34 rows, 680 linear feet, so about 38 boards 20 ft long, or 75 boards 10 ft long, with 10% waste.

How much extra decking should I buy?

About 10% for boards laid straight, and about 15% for diagonal or herringbone patterns, which have more angled cuts. Picture-frame borders add their own boards on top.

Which way do deck boards run?

Across the joists. On most attached decks the joists run out from the house, so the boards run parallel to it. Diagonal boards need joists closer together; check the decking maker's install guide.

How many screws do I need for deck boards?

Two where every board crosses a joist. For a 16 × 12 ft deck with joists 16 in apart, that's about 656 screws (2 boxes of 350), or 328 hidden fasteners for grooved composite boards, with 5% extra.

How far apart should deck boards be?

About 3/16 to 1/4 in for kiln-dried wood and most composite boards; hidden fastener clips usually set the gap for you. Wet pressure-treated boards shrink as they dry, so they go down tight or with a 1/8 in gap.

What width is a 5/4x6 deck board?

Its actual width is 5 1/2 in, the same as a 2x6. Most composite boards are about 5 1/2 in too. Measure your boards if you're not sure, and enter the actual width.